In Shakespeare's A Midsummer Night's Dream one can argue that Puck is the key character of the play. It is Puck (Robin Goodfellow), a mischievous hobgoblin or sprite, that generates much of the comedy—and Puck's character is effective in developing and driving the plot.
Perhaps most importantly, it is Puck that brings together the world of humans and fairies as he transforms Bottom's head to that of an ass. When Titania (queen of the fairies) wakens and finds Bottom there, she immediately falls in love with him (because of Cupid's "love-juice"). It is Puck who reports to Oberon how the fairy king's plan to torment his wife has played out. Puck notes:
Titania waked, and straightway loved an ass. (III.ii.35)
Oberon is very pleased:
This falls out better than I could devise. (36)
It is under these circumstances that a human is introduced to the existence of the fairy world. For Bottom it is an idealistic afternoon that he later recalls as nothing more than a strange and inexplicable dream.
Puck is important as Oberon's minion. He serves the king of the fairies—it is through his actions that Cupid's love potion is retrieved. Many comical confrontations take place between the lovers because of Puck's machinations. Puck is central to creating these conflicts.
Mistakenly, Puck has placed the "love-juice" on Lysander rather than Demetrius so that Lysander (who loves Hermia) falls in love instead with Helena. When Puck tries to fix this, he puts the potion on Demetrius' eyes, but this creates more confusion when Demetrius mistakenly falls in love with Hermia. Seeing Demetrius and Hermia together, Puck is confused; Oberon explains:
...Thou hast mistaken quite,
And laid the love-juice on some true-love's sight. (III.ii.89-90)
Oberon "fixes" Demetrius so he falls in love with Helena, but now both Demetrius and Lysander love Helena—and no one loves Hermia. Acting now as commentator, Puck suggests that he and Oberon should watch what promises to be a comical melee—and great "sport."
Shall we their fond pageant see?
Lord, what fools these mortals be! (III.ii.115-116)
Eventually, Puck must return order to everything. Oberon tells him to remove the spell from Lysander, and then to send the lovers wandering in opposite directions so there is no more confusion. The sleeping lovers are discovered together in Act Four, scene one, by Hermia's father, Egeus, and the Duke, Theseus. Demetrius no longer loves Hermia, so Theseus declares that all is well and that both couples shall marry when Theseus and Hippolyta are wed later that day.
Bottom has been returned to his natural state, and though the players present a less-than-perfect performance at the wedding, they receive the Duke's approval, a testament to further theatrical engagements.
Because of their responsibility to watch over humans (whose world suffers when Oberon and Titania fight), the king and queen of the fairies stop arguing.
Finally, Puck is the peacemaker: he apologizes for any misunderstandings, and declares that if the audience will be patient, Puck will make everything as it was—with balance restored between the fairy and human worlds.
If we shadows have offended,
Think but this, and all is mended,
That you have but slumber'd here... (V.i.418-420)
Give me your hands, if we be friends,
And Robin shall restore amends. (432-433)
Puck is key, involved in all that happens with humans and mortals. He is the most constant element in the play.